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There is nothing preventing anyone building a commercial game based on id Software's released engines, you just have to keep the engine code that you build on under the GPL.

What you describe here is the LGPL. The GPL requires you to also release the code that links to the GPL'd code under a GPL compatible license. In this case this means that not only the engine source code but also the game source code must be released under a GPL compatible license.

For example, Steel Storm is a commercial game based of the DarkPlaces engine. You can not keep the engine code proprietary, but your game data can still be proprietary if you so choose

Sure the game data can be proprietary. But the makers of Steel Storm have also not released the game source code and are in direct violation of the GPL.

What you describe here is the LGPL. The GPL requires you to also release the code that links to the GPL'd code under a GPL compatible license. In this case this means that not only the engine source code but also the game source code must be released under a GPL compatible license.

Sure the game data can be proprietary. But the makers of Steel Storm have also not released the game source code and are in direct violation of the GPL.

It is possible to get alternative licensing terms from ID Software, which would circumvent the GPL.

Steel Storm is based on the GPL engine Darkplaces, and its media license is even "almost" FOSS too with the CC-BY-NC-SA license. This makes it possible to use its engine enhancements (like a in-game editor, a really nice menu system, etc) also in other open-source games!

I think this is actually not a bad compromise between trying to cover some expenses and still making open-source games, and that commercial efforts are not strictly incompatible with FOSS game development can be discussed here) It would be cool though, if they would consider a "ransom" design also, i.e. that the game becomes completely FOSS after a certain sum of sales has been reached.